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Spacecraft Collides With Longest-Ever Comet Tail
By Maia Weinstock
Staff Writer
posted: 11:19 am ET
05 April 2000
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Sometimes in science, little surprises can have great impactsSometimes in science, little surprises can have great impacts. Just ask the astronomers who stumbled across a small blip in the data collected from the European Space Agency/NASA Ulysses spacecraft when it passed through supposedly empty space. The blip, it turns out, was the signature from an invisible comet tail, a comet tail that was more than 310.7 million miles (500 million kilometers) long -- almost double the longest comet tail previously known to exist. "This was a complete surprise," said Geraint Jones, an astronomer with the Space and Atmospheric Physics Group at Imperial College, London. "The chances of an unexpected comet tail crossing like this are slim. There are a lot of conditions that all have to be met for the comet and spacecraft to be in exactly the right places at the right times." In the April 6th edition of Nature, Jones and 11 other researchers revealed that a comet had passed through the exact position that Ulysses -- a probe orbiting the sun that studies the solar wind -- occupied on May 1 only eight days earlier. The comet in question was the Hyakutake comet, which was visible to the naked eye from Earth during the spring and summer of 1996. 
"Hyakutake was in the right place at the right time. It was perfect alignment, just incredible!"
"For the first time, weve been able to measure on-site what a comets tail is like so far from the nucleus," explained Jones. "Before, we could only guess what conditions were like there because we had to rely on ground-based observations for our information. We now have a brand new perspective on comets." Comets are small icy objects that formed at the dawn of the solar system, some 4.5 billion years ago. As a comet nears the sun, its icy core boils off, forming a cloud of dust and gas called a head, or coma. Comets become visible when sunlight reflects off this cloud. As the comet gets closer to the sun, more gas is produced. The gas and dust is pushed away by charged particles known as the solar wind, forming two tails. Dust particles form a yellowish tail, and charged gas makes a bluish ion tail. A comet's tails always point away from the sun. An unlikely find Clues about Ulysses contact with Comet Hyakutakes tail began to appear when a team of scientists who were studying charged particles emanating from the sun noticed something odd in their data. "The various chemical elements we were seeing were completely different from solar [chemicals]," said George Gloeckler, an astronomer at the University of Maryland. "Then some people on our team said, Hey, thats cometary composition, so we knew right away it was a comet." The only problem, said Gloeckler, was figuring out which comet it was. "We searched for all known comets, and of all of them, Hyakutake was in the right place at the right time," said Gloeckler. "It was perfect alignment, just incredible! It was a fantastic coincidence." 
The Ulysses probe ran into Hyakutake's tail while studying the solar wind. Meanwhile, Jones research team was studying Ulysses data about the strength and direction of the magnetic field flowing outward from the sun. As it turns out, they too saw a strange signal, indicating a comet tail. "The magnetic field lines had hairpin shapes, a bit like a herringbone pattern...this is exactly whats expected in a comets tail," said Jones. "Our data also showed that the tail was the right size to belong to Hyakutake, and that it was also oriented in the right way for it to belong to that comet. These checks made us absolutely sure that it was Hyakutakes tail that wed crossed." Comets in our future Scientists say the Ulysses-Hyakutake collision will help comet experts understand the life cycle and fate of comets, which many think of as prehistoric relics from the earliest days of the solar system. In particular, the discovery of such a long comet tail invites astronomers to believe that they may find and study past comets whose tails are still intact. "Nobody really believed that you could have a comet tail this long," said Gloeckler. "They thought it would break up, dissipate, and it would be so diluted, you wouldnt see it at all. But now with this kind of measurement, its clear that these tails stay intact far longer." Today, Comet Hyakutake is far, far away from the Earth: 1.2 billion miles (2 billion kilometers), to be exact. But astronomers speculate that there will always be new comets flying in the vicinity of our home planet. As a result, a cometary impact on Earth is always a possibility and concern. However, scientists now say they may be able to use the results from the Ulysses-Hyakutake finding to study the probability of comets hitting the Earth. "Once in a great while, a small comet actually hits the Earth," said Gloeckler. "There was one in 1908 in Siberia, but nobody much lived there. If a comet hit a populated area, it would be devastating. With this new technique, one could also do a search for small comets around Earth."
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